X.U.ExtensibleConf: Flip arguments of once(M)

This appears to be more natural. The function will most often be fixed
by the module using `XC.once` and the configuration will often be
supplied by users of those modules, so it's better to partially apply
the function first.

XMonad/Hooks/Rescreen.hs

@@ -90,7 +90,9 @@ instance Monoid RescreenConfig where
 -- Note that 'rescreenHook' is safe to use several times, 'rescreen' is still
 -- done just once and hooks are invoked in sequence, also just once.
 rescreenHook :: RescreenConfig -> XConfig a -> XConfig a
-rescreenHook = flip XC.once rescreenHook'
+rescreenHook = XC.once $ \c -> c
+    { startupHook = startupHook c <> rescreenStartupHook
+    , handleEventHook = handleEventHook c <> rescreenEventHook }
 
 -- | Shortcut for 'rescreenHook'.
 addAfterRescreenHook :: X () -> XConfig a -> XConfig a
@@ -100,10 +102,6 @@ addAfterRescreenHook h = rescreenHook def{ afterRescreenHook = h }
 addRandrChangeHook :: X () -> XConfig a -> XConfig a
 addRandrChangeHook h = rescreenHook def{ randrChangeHook = h }
 
-rescreenHook' :: XConfig a -> XConfig a
-rescreenHook' c = c{ startupHook = startupHook c <> rescreenStartupHook
-                   , handleEventHook = handleEventHook c <> rescreenEventHook }
-
 -- | Startup hook to listen for @RRScreenChangeNotify@ events.
 rescreenStartupHook :: X ()
 rescreenStartupHook = do

XMonad/Util/ExtensibleConf.hs

@@ -113,16 +113,16 @@ add x = alter (<> Just x)
 -- This can be used to implement a composable interface for modules that must
 -- only hook into xmonad core once.
 once :: forall a l. (Semigroup a, Typeable a)
-     => a -- ^ configuration to add
+     => (XConfig l -> XConfig l) -- ^ 'XConfig' modification done only once
-     -> (XConfig l -> XConfig l) -- ^ 'XConfig' modification done only once
+     -> a -- ^ configuration to add
      -> XConfig l -> XConfig l
-once x f c = add x $ maybe f (const id) (lookup @a c) c
+once f x c = add x $ maybe f (const id) (lookup @a c) c
 
 -- | Config-time: Applicative (monadic) variant of 'once', useful if the
 -- 'XConfig' modification needs to do some 'IO' (e.g. create an
 -- 'Data.IORef.IORef').
 onceM :: forall a l m. (Applicative m, Semigroup a, Typeable a)
-      => a -- ^ configuration to add
+      => (XConfig l -> m (XConfig l)) -- ^ 'XConfig' modification done only once
-      -> (XConfig l -> m (XConfig l)) -- ^ 'XConfig' modification done only once
+      -> a -- ^ configuration to add
       -> XConfig l -> m (XConfig l)
-onceM x f c = add x <$> maybe f (const pure) (lookup @a c) c
+onceM f x c = add x <$> maybe f (const pure) (lookup @a c) c

tests/ExtensibleConf.hs

@@ -22,9 +22,9 @@ spec = do
         XC.lookup (XC.add "a" (XC.add [1 :: Int] def)) `shouldBe` (Nothing :: Maybe ())
 
     specify "once" $
-        borderWidth (XC.once "a" incBorderWidth def) `shouldBe` succ (borderWidth def)
+        borderWidth (XC.once incBorderWidth "a" def) `shouldBe` succ (borderWidth def)
     specify "once . once" $
-        borderWidth (XC.once "b" incBorderWidth (XC.once "a" incBorderWidth def))
+        borderWidth (XC.once incBorderWidth "b" (XC.once incBorderWidth "a" def))
             `shouldBe` succ (borderWidth def)
 
 incBorderWidth :: XConfig l -> XConfig l